Projections

You can create a projection abstract based on single stream.

stream_name = "Customer$1"

client.publish(MoneyDeposited.new(data: { amount: 10 }), stream_name: stream_name)
client.publish(custom_event = MoneyDeposited.new(data: { amount: 20 }), stream_name: stream_name)
client.publish(MoneyWithdrawn.new(data: { amount: 5 }), stream_name: stream_name)

account_balance =
  RailsEventStore::Projection
    .from_stream(stream_name)
    .init(-> { { total: 0 } })
    .when(MoneyDeposited, ->(state, event) { state[:total] += event.data[:amount] })
    .when(MoneyWithdrawn, ->(state, event) { state[:total] -= event.data[:amount] })

account_balance.run(client) # => {total: 25}

In order to narrow the results, simply pass event_id to run method.

account_balance.run(client, start: custom_event.event_id) # => {total: -5}

You may also subscribe one handler to multiple events by passing an array to #when method:

account_cashflow =
  RailsEventStore::Projection
    .from_all_streams
    .init(-> { { total: 0 } })
    .when([MoneyDeposited, MoneyWithdrawn], ->(state, event) { state[:total] += event.data[:amount] })

account_cashflow.run(client) # => {total: 35}

Projection based on multiple streams

client.publish(MoneyDeposited.new(data: { amount: 15 }), stream_name: "Customer$1")
client.publish(MoneyDeposited.new(data: { amount: 25 }), stream_name: "Customer$2")
client.publish(custom_event = MoneyWithdrawn.new(data: { amount: 10 }), stream_name: "Customer$3")
client.publish(MoneyWithdrawn.new(data: { amount: 20 }), stream_name: "Customer$3")

account_balance =
  RailsEventStore::Projection
    .from_stream("Customer$1", "Customer$3")
    .init(-> { { total: 0 } })
    .when(MoneyDeposited, ->(state, event) { state[:total] += event.data[:amount] })
    .when(MoneyWithdrawn, ->(state, event) { state[:total] -= event.data[:amount] })

account_balance.run(client) # => {total: -15}

In order to narrow the results, you have to pass array with event_id for each stream or nil to start from beggining of the stream. So, in example below we start from beggining of stream for Customer$1 and custom_event.event_id for Customer$3.

account_balance.run(client, start: [nil, custom_event.event_id]) # => {total: -5}

Projection based on all streams

client.publish(MoneyDeposited.new(data: { amount: 10 }), stream_name: "Customer$1")
client.publish(MoneyDeposited.new(data: { amount: 10 }), stream_name: "Customer$2")
client.publish(custom_event = MoneyDeposited.new(data: { amount: 10 }), stream_name: "Customer$3")
client.publish(MoneyWithdrawn.new(data: { amount: 20 }), stream_name: "Customer$4")

account_balance =
  RailsEventStore::Projection
    .from_all_streams
    .init(-> { { total: 0 } })
    .when(MoneyDeposited, ->(state, event) { state[:total] += event.data[:amount] })
    .when(MoneyWithdrawn, ->(state, event) { state[:total] -= event.data[:amount] })

account_balance.run(client) # => {total: 10}
account_balance.run(client, start: custom_event.event_id) # => {total: -20}